This is a portal from the twilight forest mod. It was made in snapshot 14w27b with the /execute and detect feature. This detects whether the diamond is above hidden gold blocks under the portal area. It then kills the diamond item and sets the portal alight. You may see a scoreboard flash upon doing so. That's the portalLit scoreboard, which tells the portal to light itself.
I hope you like it! Comment below what item, contraption or structure from a mod you want to see!

P.S. There can be no other item in the pool when the diamond is thrown in!